Multi-sensor system for airborne geophysical prospecting and method

ABSTRACT

A multi-sensor electromagnetic (EM) system and method for measuring gradients of EM signals. The multi-sensor EM system includes a frame; a transmitter device attached to the frame and configured to generate a primary EM field; a receiver device attached to the frame and configured to record a secondary EM field generated by the earth after being excited by the primary EM field; and a gradient sensor device attached to the frame and configured to record a gradient of the secondary EM field.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/966,059, filed Dec. 11, 2015, which is related to, and claimspriority from, U.S. Provisional Patent Application Ser. No. 62/092,937filed Dec. 17, 2014, entitled “APPARATUS FOR AIRBORNE EM SURVEYING USINGTHREE COMPONENT TRANSMITTERS, RECEIVERS AND GRADIENT SENSORS”, thedisclosure of which is incorporated here by reference.

BACKGROUND Technical Field

Embodiments of the subject matter disclosed herein generally relate tomethods and systems for generating and/or measuring electromagnetic (EM)fields and, more particularly, to mechanisms and techniques forenhancing detection of near surface, lateral or vertical conductivetargets.

Discussion of the Background

EM surveying is a method of geophysical exploration to determine theproperties of a portion of the earth's subsurface, information that isespecially helpful in the oil and gas industry and the mining industry.EM surveys may be based on a controlled source that generates a primaryEM field which carries EM energy into the earth, which induces eddycurrents in the earth. The eddy currents generate a secondary EM fieldor ground response. By measuring the secondary field with an EMreceiver, it is possible to estimate the depth and/or composition of thesubsurface features. These features may be associated with subterraneanhydrocarbon deposits.

A schematic airborne EM survey system 100 generally includes, asillustrated in FIG. 1, a transmitter 102 for generating the primary EMfield 104 that is directed toward the earth. When primary EM field 104enters the ground 108, it induces eddy currents 106 inside the earth.These eddy currents 106 generate a secondary electromagnetic field orground response 110. An EM receiver 112 then measures the response 110of the ground. Transmitter 102 and receiver 112 may be connected to anaircraft 114 so that a large area of the ground is swept. Receiver 112may be located concentric and/or coplanar with transmitter 102. A secondreceiver 113 may also be added.

However, the existing EM surveying systems are not sensitive to nearsurface formation detection as the signals usually associated with suchformation are buried in either ambient or geologic noise. Thus, there isa need to have a new system that is capable of measuring signalsassociated with near surface formation detection.

SUMMARY

One or more of the embodiments discussed herein illustrate a newmulti-sensor EM system that is capable of simultaneously recording notonly EM signals associated with the underground formations, but also agradient of the secondary EM fields.

According to one embodiment, there is a multi-sensor electromagneticsystem for measuring EM signals. The multi-sensor EM system includes aframe, a transmitter device attached to the frame and configured togenerate a primary EM field, a receiver device attached to the frame andconfigured to record a secondary EM field generated by the earth afterbeing excited by the primary EM field, and a gradient sensor deviceattached to the frame and configured to record a gradient of thesecondary EM field.

According to another embodiment, there is an electromagnetic surveyingsystem for measuring EM signals. The EM surveying system includes anairborne carrier configured to fly over ground, a multi-sensor EM systemconfigured to generate a primary EM field and to record a secondary EMfield generated by the earth as a response to the primary EM field, anda towing system connecting the airborne carrier to the multi-sensor EMsystem. The multi-sensor EM system is also configured to measure agradient of the secondary EM field.

According to still another embodiment, there is a method for recordingelectromagnetic signals during an airborne EM survey. The methodincludes flying a multi-sensor EM system, generating a primary EM fieldwith a transmitter device attached to a frame of the multi-sensor EMsystem, recording, with a receiver device attached to the frame, asecondary EM field generated by the earth after being excited by theprimary EM field, and recording, with a gradient sensor device attachedto the frame, a gradient of the secondary EM field.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate one or more embodiments and,together with the description, explain these embodiments. In thedrawings:

FIG. 1 is a schematic diagram of an EM acquisition system;

FIG. 2 is a schematic diagram of a multi-sensor EM acquisition system;

FIG. 3 is a general view of the multi-sensor EM system;

FIG. 4 illustrates the X axis coils corresponding to a transmitter,receiver and gradient sensor of the multi-sensor EM system;

FIG. 5 illustrates the transmitter and receiver coil position relativeto each other;

FIG. 6 illustrates the arrangement of the transmitter and gradientsensor coils relative to each other;

FIG. 7 illustrates the Y axis coils corresponding to a transmitter,receiver and gradient sensor of the multi-sensor EM system;

FIG. 8 illustrates the Z axis coils corresponding to a transmitter,receiver and gradient sensor of the multi-sensor EM system;

FIG. 9 is a schematic diagram of a multi-sensor EM system towed with anairborne carrier;

FIG. 10 shows the multi-sensor EM system disposed over a hybrid airvehicle;

FIG. 11 is a flowchart of a method for collecting EM data with amulti-sensor EM system; and

FIG. 12 is a schematic diagram of a processing unit that coordinates thetransmitter, receiver and gradient sensor devices.

DETAILED DESCRIPTION

The following description of the embodiments refers to the accompanyingdrawings. The same reference numbers in different drawings identify thesame or similar elements. The following detailed description does notlimit the invention. Instead, the scope of the invention is defined bythe appended claims. The following embodiments are discussed, forsimplicity, with regard to the terminology and structure of amulti-sensor EM system having three-component sensors. However, theembodiments to be discussed next are not limited to such sensors, butthey may work with fewer components per sensor.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with an embodiment is included in at least oneembodiment of the subject matter disclosed. Thus, the appearance of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout the specification is not necessarily referring to the sameembodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

According to an embodiment, a multi-sensor EM system includes at leastone three-component transmitter and a multi-sensor. The multi-sensor mayinclude a three-component receiver and a three-component gradientsensor. The transmitter and multi-sensors are attached to a platformthat is suitable for airborne surveys. In one application, the gradientsensor includes gradient coils placed symmetrically about thetransmitter coils to cancel the primary field, because the sum of thetwo coils will have no primary field. This allows greater sensitivity tothe secondary field gradient. Details of the multi-sensor EM system arenow discussed with regard to the figures.

FIG. 2 shows one embodiment of a multi-sensor EM system 200 that isdesigned for airborne geophysical exploration. Multi-sensor EM system200 includes a rigid frame 202 that hosts a three-component transmitterdevice 210, a three-component receiver device 220 and a three-componentgradient sensor device 230. Frame 202 may be made out of tubes 204 asillustrated in FIG. 3. Tubes 204 are hollow inside and configured toaccommodate one or more wires that form the transmitter, receiver and/orgradient sensor coils. Tubes 204 are connected to each other to form acircle, hexagon or any desired shape. Tubes 204 may be made of plasticor other insulator material. The tubes are light but rigid, so that theydo not allow a deformation of the coils located inside the tubes. Thetubes are attached to each other as known in the art.

Returning to FIG. 2, a local control device 240 may be located on theframe 202 and it is electrically connected to the transmitter, receiverand gradient sensor devices. Local control device 240 may communicatewith a global control device (not shown, but located, for example, onthe airborne carrier) for coordinating the generation of the primary EMfields in the transmitter device and for instructing the receiver andthe gradient sensor devices when to record the secondary EM signals. Thelocal control device may be configured to send commands to thetransmitter device to sequentially generate magnetic fields in threemutually orthogonal orientations. These magnetic fields may becalculated and generated in such a way that a combination of the fieldsfrom multiple transmitters enhances the fields at specific locations andin a specific orientation. This combination has the potential toincrease the signal-to-noise thereof, which is desirable. In oneembodiment, the local control device is configured to instruct thethree-component transmitter device to continuously generate arbitrarywaveforms at similar or unique pulse repetition rates. Those skilled inthe art would understand that the local and/or global control devicesmay be configured to instruct the three-component transmitter device togenerate any desired shape EM signal.

In addition, the local and/or global control devices may be configuredto combine the various EM signals received from the receiver device andthe gradient sensor device to enhance the near surface, lateral and/orvertical conductive underground target formations. Such algorithms areknown in the art and thus, they are not discussed herein.

Each of the transmitter device 210, receiver device 220 and gradientsensors device 230 is now discussed in detail. FIG. 4 shows thelongitudinal components of each of the devices 210, 220 and 230. Morespecifically, FIG. 4 shows the longitudinal transmitter coil 412X of thetransmitter device 210, the longitudinal receiver coil 422X of thereceiver device 220, and the longitudinal gradient sensor coil 432X ofthe gradient sensor device 430. All these components are perpendicularto the X axis, which may be considered to indicate the travel directionof the multi-sensor EM system's carrier. FIG. 4 shows that longitudinaltransmitter coil 412X is concentric and between the two longitudinalreceiver coils 422X.

The specific arrangement of the receiver coils is called concentricnull-coupled receiver and it is illustrated in more detail in FIG. 5.FIG. 5 shows that the receiver coil 422X has in fact two concentriccoils located in a same plane and these two coils are also concentricand located in a same plane with the transmitter coil 412X. Currentsflowing through the two concentric receiver coils have oppositedirections. This arrangement of the receiver coils containssubstantially equal and opposite components of the transmitted field.This provides an overall increase in sensitivity for the secondaryfields of interest. The system may also include one or more adjustablecomponents to fine tune the primary field nulling.

FIG. 4 also shows the gradient sensor coil 432X, which includes twoparallel elements (coils) 432A and 432B located symmetrically about eachtransmitter coil 412X. Each coil pair 432A and 432B is wound in series,opposite to each other, as illustrated in FIG. 6, to provide a signalthat is substantially devoid of the primary EM field and ambient noise,but it is sensitive to the spatial gradient of the secondary fields ofinterest for each axis. Currents flowing through the coil pair 432A and432B have opposite directions. This allows for detection of near surfacestructures that might otherwise be buried in either ambient or geologicnoise.

FIG. 7 shows the relative positions of the transmitter component 412Y,receiver component 422Y and gradient sensor component 432Y along the Yaxis, which is substantially identical to the structure illustrated inFIG. 4. FIG. 8 shows the Z components 412Z, 422Z and 432Z of thetransmitter, receiver and gradient sensor devices, respectively. Notethat FIG. 8 also shows some of the tubes 804 forming the frame thatsupports the coils discussed above.

The structural components (frame 202, tubes 804 and other connectingdevices not shown) supporting the three axis transmitters, receivers andgradient coils may be assembled as a stand-alone unit of arbitrary shapeto be towed under a carrier. FIG. 9 shows such a system 900 having acarrier 902 that tows with a towing system 904 (e.g., ropes and/orcable) a multi-sensor EM system 906. System 906 includes at least atransmitter device 910, receiver device 920 and gradient sensor device930 as discussed in the previous embodiments. Carrier 902 may be a lightweight helicopter or a lighter than air vehicle or a hybrid air vehicleor any aircraft. A hybrid air vehicle is defined by two components: apropelling mechanism for generating thrust and an enclosure that housesa lighter than the air gas that provides positive buoyancy. In this way,the thrust generated by the propelling mechanism is not used to maintaina given altitude of the hybrid vehicle. Such vehicles can cover a largerphysical area resulting in greater sensitivity and low flight speeds,which results in increasing data density and lower overall noise levels(no vibrating engines).

In one embodiment, the multi-sensor EM system is substantially composedof composite structures formed in such a way as to provide fundamentallya symmetric three component transmitter. One transmitter is verticallyperpendicular to the flight direction (X), one transmitter ishorizontally perpendicular to the flight direction and one transmitteris horizontally parallel to the flight direction. The transmitters mayhave the same physical area. However, inflight loading related toairflow over the structures limits the practical size of the X and Ytransmitters to approximately ⅕ to ½ the area of the Z looking coil. Thetransmitter device structure may be tubular or a composite 1-beam asdisclosed in U.S. Pat. No. 7,646,201, the entire content of which isincorporated herein by reference. The transmitter coils are collocatedforming a substantially rigid structure. Typical transmitter pulserepetition frequencies include 30 Hz for each axis, if firedsequentially, or 29.5 Hz, 30.0 Hz and 31.0 Hz in each of the X, Y and Zaxis. In some cases, it is possible transmitting similar waveforms oneach axis at the same pulse repetition rate. In one embodiment, it maybe desirable to independently control the power levels of each axis todirect the primary magnetic field towards a desired target (e.g., alonga chosen direction). The power levels may also be controlled in acontinuously varying fashion to essentially sweep the resulting magneticfield to ensure maximum coupling of the directed field with a conductivetarget of arbitrary geometry. The power levels may be controlled by thelocal and/or global control devices discussed above. This approach maybe particularly useful in the field of unexploded ordinance mapping.

FIG. 10 illustrates an embodiment in which carrier 902 includes threetransmitting coils 910X, 910Y and 910Z that generate EM signals forpenetrating the earth. Carrier 902 is a hybrid air vehicle. Each of thetransmitting coils includes a coil of wire extending around a gasenvelope 903 of the carrier. Gas envelope 903 is configured to host thelighter than air gas. For example, a first transmitting coil 910X isoriented in a vertical plane extending around the gas envelope of thecarrier 902, transversely to the longitudinal axis of the carrier, andthereby, having a dipole moment along the X-axis. Coil 910X may belocated generally midway between the nose and the tail of the carrier tomaximize its area. However, coil 910X may be located anywhere along thecarrier. A second transmitting coil 910Y is oriented in a vertical planeextending around the envelope 903, from the nose to the tail, with adipole moment on the Y-axis. A third transmitting coil 910Z is orientedin a horizontal plane extending around the gas envelope of the carrier,from the nose to the tail, with a dipole moment on the Z-axis.

According to another embodiment, the transmitting coils 910X-Z aregenerally elliptical in shape. Alternatively, the transmitting coils canbe of other shapes that can be accommodated by the carrier. In oneembodiment, the coils follow the shape of the envelope 903. Receivercoils 920X-Z, which measure the ground response due to the excitationfrom the transmitting coils, may be located next to the transmittingcoils, as illustrated in FIG. 10. The use of one or more three-componentreceiver(s) 920, along with the orientations of the transmitting coils910X-Z thereby allow collection of data in three different orthogonaldirections, being nominally vertically perpendicular to the direction offlight, horizontally parallel to the direction of flight andhorizontally perpendicular to the direction of flight. The gradientsensor device 930 may also be located on the carrier. For simplicity,FIG. 10 shows only the X components 930A and 930B of the gradient sensordevice 930.

The multi-sensor EM system discussed herein takes 12 measurements of theEM field for each transmitter firing sequence. These measurementsadvantageously can be combined to enhance near surface, lateral orvertical conductive targets. However, in one embodiment, themulti-sensor EM system may have less than 12 measurements, i.e., eitherone of the transmitter, receiver and gradient sensor devices may haveless than three components. For example, it is possible to build asingle axis system that includes just gradient coils. It is alsopossible to build a system that includes a transmitter and a concentricnull coupled receiver, which is consistent in function with all otherTDEM systems assuming a Z orientation.

According to still another embodiment, an additional payload that can becarried by hybrid air vehicle 902, as compared to the payload of atraditional aircraft or helicopter, may also be used to carry a largeauxiliary power unit (not shown) with sufficient electrical capacity toincrease the output power of one or more of the transmitter coils, e.g.,by a factor of twenty or more. For example, the transmitter power of thetransmitter device 910 may be as much as 40,000,000 amp-metres square(Am²) whereas the most powerful system currently mounted on helicopteror aircraft is in the order of 2,000,000 Am². The null-coupled receiverdevice allows for increases in transmitter moment without sacrificingreceiver sensitivity.

A method for recording secondary magnetic fields with an airbornemulti-sensor EM system is now discussed with regard to FIG. 11. Themethod includes a step 1100 of flying a multi-sensor EM system, a step1102 of generating a primary EM field with a transmitter device attachedto a frame of the multi-sensor EM system, a step 1104 of recording, witha receiver device attached to the frame, a secondary EM field generatedby the earth after being excited by the primary EM field, and a step1106 of recording, with a gradient sensor device attached to the frame,a gradient of the secondary EM field. The multi-sensor EM system may beany of the systems discussed above.

There are many possible implementations of the multi-sensor EM systemdiscussed above. The multi-sensor EM system may include, in addition tothe components discussed above, many peripheral sensors to determine theposition or orientation or state of the electromagnetic measurement,such as a Global Positioning System (GPS), radar or laser altimeter,gyroscopes or inclinometers measuring transmitter or sensor positions,thermometers, or other sensors measuring other geophysical data (such asradar or laser for topography, gravity or gradiometers sensors,spectrometer sensors, magnetometers to measure the ambient earthmagnetic field, etc.). Consequently, there are also many differentmethods to record, process, combine and control all of these signals andsensors.

As also will be appreciated by one skilled in the art, the exemplaryembodiments may be embodied in a processing unit 1200, as illustrated inFIG. 12. Processing unit 1200 includes a processor 1202 that isconnected through a bus 1204 to a storage device 1206. Processing unit1200 may also include an input/output interface 1208 through which datacan be exchanged with the processor and/or storage device. For example,a keyboard, mouse or other device may be connected to the input/outputinterface 1208 to send commands to the processor and/or to collect datastored in storage device or to provide data necessary to the processor.In one application, the processor combines the 12 measurements forgenerating, for example, a raw image of the underground formations.Also, the processor may be used to calculate, for example, the positionsof the coils. Results of this or another algorithm may be visualized ona screen 1210. The method discussed above may be implemented in awireless communication device or in a computer program product.Accordingly, the exemplary embodiments may take the form of an entirelyhardware embodiment or an embodiment combining hardware and softwareaspects. Further, the exemplary embodiments may take the form of acomputer program product stored on a computer-readable storage mediumhaving computer-readable instructions embodied in the medium. Anysuitable computer-readable medium may be utilized, including hard disks,CD-ROMs, digital versatile discs (DVD), optical storage devices ormagnetic storage devices such as a floppy disk or magnetic tape. Othernon-limiting examples of computer-readable media include flash-typememories or other known types of memories.

For greater clarity, the figures used to help describe the invention aresimplified to illustrate key features. For example, figures are not toscale and certain elements may be disproportionate in size and/orlocation. Furthermore, it is anticipated that the shape of variouscomponents may be different when reduced to practice, for example. Thepatentable scope of the subject matter is defined by the claims, and mayinclude other examples that occur to those skilled in the art. Suchother examples are intended to be within the scope of the claims. Thoseskilled in the art would appreciate that features from any embodimentsmay be combined to generate a new embodiment.

The disclosed embodiments provide a method and multi-sensor EM systemcapable of recording gradient EM signals indicative of a near surfaceformation, in addition to the traditional EM signals. It should beunderstood that this description is not intended to limit the invention.On the contrary, the exemplary embodiments are intended to coveralternatives, modifications and equivalents, which are included in thespirit and scope of the invention as defined by the appended claims.Further, in the detailed description of the exemplary embodiments,numerous specific details are set forth in order to provide acomprehensive understanding of the claimed invention. However, oneskilled in the art would understand that various embodiments may bepracticed without such specific details.

Although the features and elements of the present exemplary embodimentsare described in the embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the embodiments or in various combinations with or withoutother features and elements disclosed herein.

This written description uses examples of the subject matter disclosedto enable any person skilled in the art to practice the same, includingmaking and using any devices or systems and performing any incorporatedmethods. The patentable scope of the subject matter is defined by theclaims, and may include other examples that occur to those skilled inthe art. Such other examples are intended to be within the scope of theclaims.

What is claimed is:
 1. A multi-sensor electromagnetic (EM) system formeasuring EM signals, the multi-sensor EM system comprising: a frame; atransmitter device attached to the frame and configured to generate aprimary EM field, the transmitter device including three transmittercoils perpendicular to each other; a receiver device attached to theframe and configured to record a secondary EM field generated by theearth after being excited by the primary EM field; a gradient sensordevice attached to the frame and configured to record a gradient of thesecondary EM field, the gradient sensor including at least one gradientcoil having two parallel coils that sandwich one of the transmittercoils; and a controller attached to the frame and electrically connectedto the transmitter device, the receiver device, and the gradient sensordevice, wherein the controller instructs each of the three transmittercoils to generate a magnetic field with a given pulse repetitionfrequency.
 2. The system of claim 1, wherein the controller instructsthe three transmitter coils to generate the magnetic fieldssequentially.
 3. The system of claim 2, wherein the given pulserepetition frequency is common for the three transmitter coils.
 4. Thesystem of claim 1, wherein the controller instructs the threetransmitter coils to generate a continuous arbitrary waveform.
 5. Thesystem of claim 4, wherein the given pulse repetition frequency isunique for the three transmitter coils.
 6. The system of claim 4,wherein the given pulse repetition frequency is similar for the threetransmitter coils.
 7. The system of claim 1, wherein the controllercontrols a power level of each of the three transmitter coilsindependent of each other.
 8. The system of claim 1, wherein thereceiver device includes three receiver coils perpendicular to eachother, and the gradient sensor device includes three gradient coilsperpendicular to each other.
 9. The system of claim 1, wherein thegradient sensor device includes three gradient coils perpendicular toeach other.
 10. A method for recording electromagnetic (EM) signalsduring an airborne EM survey, the method comprising: flying amulti-sensor EM system; generating a primary EM field with a transmitterdevice attached to a frame of the multi-sensor EM system, wherein acontroller, which is attached to the frame, instructs each of threetransmitter coils of the transmitter device to generate a magnetic fieldwith a given pulse repetition frequency; recording with a receiverdevice attached to the frame, a secondary EM field generated by theearth after being excited by the primary EM field; and recording with agradient sensor device attached to the frame, a gradient of thesecondary EM field, wherein the gradient sensor includes at least onegradient coil having two parallel coils that sandwich one of thetransmitter coils.
 11. The method of claim 10, wherein the transmitterdevice includes three transmitter coils perpendicular to each other.